The present invention relates to an optical disc apparatus that is able to use a plurality of kinds of optical discs whose cover layers have different thickness. Particularly, the present invention relates to an optical disc apparatus that includes a fixed light source portion and a movable objective lens portion separated from the fixed light portion. In this specification, such an optical system (i.e., one which has separated light portion and optical system) will be referred to as a separate-type optical system. The objective lens portion moves along a radial direction of the optical disc independently of the light source portion.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer. A laser beam emerged from an optical pick-up is converged to form a beam spot on the information layer through the cover layer. A turntable of an optical disc apparatus rotates the optical disc mounted thereon, and the optical pick-up reproduces the recorded signal from the optical disc or records the information onto the optical disc. Due to difference of thickness of the cover layer, the position of the information layer with respect to the turntable varies, which changes the distance between the optical pick-up and the information layer.
Namely, the thicker the cover layer is, the farther the distance to the beam spot from the optical pick-up is. For example, since the cover layer of a compact disc (CD) or that of a CD-R has the thickness of 1.2 mm, and the thickness of the cover layer of a digital versatile disc (DVD) is 0.6 mm, the optical pick-up is required to move the beam spot away from the optical pick-up by 0.6 mm in the cover layer (0.4 mm in air) when the DVD is replaced with the CD or the CD-R.
There are two methods to move the beam spot along an optical axis direction. The first method changes the divergence of the laser beam incident onto the objective lens, which is equivalent to a change of an object distance. For instance, when the distance between the laser source and a collimator lens changes, the object distance changes. The second method moves the objective lens along the optical axis while keeping the incident laser beam parallel.
In the first method, spherical aberration sharply varies in undercorrected direction as the divergence becomes larger (the object distance decreases), which disturbs wave front of the laser beam. Thus, the diameter of the beam spot increases, which prevents the optical disc apparatus from reproducing the recorded information from the optical disc. Further, the separate type optical disc apparatus, which drives the movable objective lens portion along a radial direction of the optical disc independently of the fixed light source portion, is preferable for use in a computer to enable a high-speed access. However, in the separate type optical disc apparatus, light amount incident on the objective lens varies according to the movement of the objective lens portion when the incident light is divergent beam. Therefore, the separate type optical disc apparatus cannot adopt the first method.
In the second method, although a paraxial beam spot moves as the objective lens is moved, the change of the thickness of the cover layer changes spherical aberration. Since the cover layer is a plane parallel plate disposed in the convergent light beam, the spherical aberration varies in overcorrected direction as the thickness of the cover layer increases. Therefore, if the optical pick-up only moves the objective lens when the disc is replaced, wave front aberration of the laser beam is deteriorated, thereby the diameter of the beam spot increases.
It is therefore an object of the present invention to provide a separate type optical disc apparatus, which is capable of reducing spherical aberration for a plurality of kinds of the optical discs whose cover layers are different in thickness.
For the above object, according to the present invention, there is provided an improved optical disc apparatus, which includes a fixed portion having a light source portion and a movable portion that holds an objective lens and moves along a radial direction of an optical disc independently of the fixed portion, and which adopts the following constructions (a), (b), (c) and (d) to use a first optical disc having a first cover layer and a second optical disc having a second cover layer thicker than the fist cover layer.
(a) The fixed portion includes the light source portion for selectively emitting a first laser beam having a first wavelength and a second laser beam having a second wavelength longer than the first wavelength, and a collimator lens for collimating the first and second laser beams.
(b) The movable portion is provided with the objective lens that includes a positive refractive lens and a diffractive lens structure formed on at least one surface of the refractive lens. and an actuator that drives the objective lens for focusing.
(c) The diffractive lens structure has a plurality of concentric ring-shaped steps to have wavelength dependence such that spherical aberration varies in the undercorrected direction as wavelength of incident light increases.
(d) The objective lens converges the first laser beam of a predetermined diffraction order onto an information layer of the first optical disc, and converges the second laser beam of the identical diffraction order onto an information layer of the second optical disc.
The first laser beam having the first wavelength is used for the first optical disc having the first cover layer. It is assumed that the spherical aberration is compensated in this condition. When the first optical disc is replaced with the second optical disc having the second cover layer in the as-is status, that is, when only the thickness of the cover layer varies, the spherical aberration varies in overcorrected direction. Thus, the wavelength dependence described in (c) is given to the diffractive lens structure, and the second laser beam having the second wavelength is used for the second optical disc. The diffractive lens structure changes the spherical aberration in the undercorrected direction for the second laser beam as compared with that for the first laser beam, which cancels the change of the spherical aberration in the overcorrected direction due to the addition of the cover layer thickness.
An additional optical path length added by a diffractive lens structure is expressed by the following optical path difference function "PHgr"(h):
"PHgr"(h)=(P2h2+P4h4+P6h6+. . . )xc3x97xcex
where P2, P4 and P6 are diffractive coefficients of second, fourth and sixth orders, h is a height from the optical axis and xcex is wavelength of incident light.
The objective lens according to the present invention may satisfy the following condition (1);
xe2x88x9215 less than "PHgr"(h45)/xcexxe2x88x92P2xc3x97(h45)2 less than xe2x88x927xe2x80x83xe2x80x83(1)
where h45 is the height from the optical axis of a point where a light ray whose NA is 0.45 intersects the diffractive lens structure.
Further, since the laser beam becomes a parallel beam between the collimator lens and the objective lens, the optical path length thereof can be determined without restraint. Therefore, a beam shaping prism can be located in the optical path.